MS Exchange mailbox

I administer the MS Exchange server in our network. When I check the mailbox resources, I find that there are many users who have a very large volume of emails. I would like to know if there is a way for me to delete these messages from the mailboxes as a maintenance procedure. I tried setting a limit to the storage space in the mailboxes but still the volume of emails in the mailboxes do not change. I hope there’s someone there who could help. Thanks.